Gas tank hook up 71demon

71 had two different emission/venting setups for the tank. It depends on the build date on what system your car came with. So if you look up a fuel tank for a 71 you will see an option for "with EEC" and without. I dont recall if it has an additional fitting on the sending unit but it does have many additional fittings on the tank. I also know the cap is different. It sounds like you might be pulling a vacuum on the tank. I agree with leaving the cap loose and give it another run to see if the problem clears up.
As far as hose routing, the FSM should lay all that out.
1971 DODGE DART 5.6L 340cid V8 Fuel Tank | RockAuto